In an electrophotographic process, electrostatic latent images are first formed on a photosensitive member of a photoconductor by various exposure means. The electrostatic latent images are then developed using an electrostatic image developer, or the so-called toner, and are subsequently transferred onto a transfer material such as paper or transparent films to produce positive images. Typically a pressure heating system utilizing hot rollers is applied to fix the toner image on the transfer material and produce permanent prints. Finally, the toner remaining on the photosensitive member is removed to prepare the photosensitive member for next electrophotographic operation.
Toner and photoconductor are the two most important expense items in copiers and/or laser printers. The toners that are commonly used in the electrophotographic process comprise a resin binder, a pigment such as carbon black or Fe.sub.3 O.sub.4, a charge control agent (CCA), a low molecular weight polymeric wax type release agent such as low molecular weight polyethylene or low molecular weight polypropylene. The amount of resin binder typically constitutes about 50-90% of the total toner composition. In addition to its function as a binder to bind together all the components of a toner composition, a resin binder also imparts many of the important printing qualities that can significantly affect the toner performance in the electrophotographic process. The qualities, which include thermal fixing property, anti-offseting characteristics, humidity resistance, optical (toner) density, uniformity, storageability, etc, are the focus of extensive research efforts in attempts to improve the electrophotographic process. With the proliferation of high performance personal computers which has brought forth a strong demand for laser printers, and the increased availability of high speed copying machines, both of which result in significant increase in toner consumption, the need to develop improved toners further escalates.
U.S. Pat. No. 4,340,660 ('660 patent) discloses a toner composition comprising a first crosslinked vinyl polymer having a gel content of 50-99% and a second cross-linked vinyl polymer having a gel content of 0-10%. The first vinyl polymer was prepared from an emulsion polymerization technique and the second vinyl polymer was prepared from a solution polymerization technique. The two polymers were mixed in a ball-mill followed by melting and kneading by a roll mill to become a resin binder for use in a toner composition. The polymer blend disclosed in the '660 patent provides good thermal fixing property and anti-offseting characteristics; however, it suffers from a quality control problem due to the complexity of the blending process.
U.S. Pat. No. 4,486,524 ('524 patent) discloses a toner comprising a resin component which does not require a blend as does the '660 patent. In the '524 patent, the resin was prepared using a bulk polymerization technique at low temperature low initiator concentration condition, followed by a solution polymerization with the addition of fresh reaction monomers containing a higher concentration of initiator. The resin prepared from the '524 patent is a mixture of two polymers, one has a molecular weight between 2,000 and 30,000 and the other between 100,000 and 500,000.
U.S. Pat. No. 4,652,511 ('511 patent) discloses a process for producing a resin composition by first preparing a high-molecular-weight polymer having a weight average molecular weight of at least 500,000 using an emulsion polymerization technique. The reaction product, being in a latex form and containing high-molecular-weight polymer, was then subject to a suspension polymerization reaction to obtain a polymer mixture containing the high-molecular-weight polymer and a low-molecular-weight polymer. The low-molecular-weight polymer has a weight average molecular weight of between 5,000 and 50,000.
U.S. Pat. No. 4,966,829 ('829 patent) discloses a toner composition containing a resin binder which has a bimodal molecular weight distribution, with a main peak in the molecular weight range of 1,000 to 25,000 and a shoulder in the molecular weight range of 2,000 to 150,000. In the '829 patent, the resin binder was prepared in a two-step process by first producing a low-molecular-weight polystyrene using a solution polymerization technique. The low-molecular-weight polystyrene was then dissolved in a suspending monomer solution to effect a suspension polymerization reaction.
All the resin binders disclosed in '524, '511 and '829 patents mentioned above avoid the blending step required in the '660 patent; however, they involve a multi-step process that is necessarily cumbersome and time-consuming. Furthermore, because all of these process involve two or more different polymerization mechanisms, the amount of liquid wastes generated therefrom can be quite significant. And the disposal of spent solvent could also present an environmental problem.
Anti-offseting characteristic is also an important consideration for electrophotographic toners. One approach to improve the offsetting problem is to apply a silicone oil over the surface of the fixing roller, in a process disclosed in U.S. Pat. No. 4,579,908. The extra step of applying the silicone oil complicates the design of a copy machine or a laser printer. The need to use silicone oil in the copying or printing machine also introduces the problems of environment pollution. Therefore, it is preferable to develop toners with improved anti-offsetting property that do not require the need of applying silicone oil.
Japanese Pat. Pub. No. 62-96672 ('672 reference) discloses a toner composition containing particulate flow aids whose surface is coated with silicone oil. In the preparation of the toner powder, it typically goes through the steps of blending, milling, coarse-pulverizing and find-pulverizing to obtain toner powder having a particle size between 5-30 .mu.m. To obtain favorable rheological properties and electrostatic chargeability thereof, about 0.1 to 2 wt % of flow aids are often added to the toner powder. Commonly used flow aids include silicon dioxide (SiO.sub.2), titanium dioxide (TiO.sub.2), aluminum oxide (Al.sub.2 O.sub.3), etc., which are uniformly dispersed onto the surface of the toner powder after thorough blending. After long-term storage or exposure to high-humidity environment, the flow aids often absorb moisture which would adversely affect the rheological property and electrostatic chargeability of the toner powder to which they are attached.
The '672 reference was intended to improve the inadequate hydrophobic characteristic of the flow aids described above by applying thereon a layer of silicone oil. However, in actual situations, other components of the toner, such as resin binder, pigment, charge control agent, etc, could also absorb moisture, in addition to the flow aids. And the surface area covered by the hydrophobically treated flow aids only comprises a relatively smaller portion of the total surface area of the toner powder. Therefore, even after the hydrophobic treatment of the flow aids, the problem regarding the humidity resistance of the toner still remains largely unsolved. Furthermore, the hydrophobic treatment disclosed in the '672 reference created a new problem in that the primary particles of the flow aids often become primary aggregates or bulk aggregates after the silicone oil treatment regardless of whether spraying or soaking method is used to effect the coating layer. This prevents a desired uniform blending between the toner powder and the flow aids. Consequently, there also remains a need to develop improved toners at least with regard to the long-term storageability and tolerance under a high humidity environment.
Toner compositions are also taught in a number of other U.S. Patents, such as U.S. Pat. Nos. 5,162,188, 5,166,026, 5,166,029, 5,169,736, 5,171,653, 5,173,387, 5,175,070, and 5,175,071.